Heretofore, in a multi-stage or section bandpass filter a plurality of di-electric resonators are integrally formed as shown in FIG. 17. The materials of the resonators are (ZnSn)TiO.sub.4 series ceramics, BaO--PbO--Nd.sub.2 O.sub.3 --TiO.sub.2 series ceramics etc. This filter is small but has the following defects.
1. Frequency adjustment due to the disorder of sintering (calcination) is required and for this purpose, hot side or earth side electrodes 13 or 14 (FIG. 17) of copper or silver are provided on the open face of the resonator. These hot side or earth side electrodes 13 or 14 are trimmed by scraping or shaving with a laser light, by sandblasting or a by cutting with diamond cutter. But laser trimming is expensive; sandblasting needs setting or adjusting of trimming time according to the thickness of the electrode and the work is not constant, and diamond cutter blades become clogged or choked with metal scraped from the electrode and needs troublesome maintenance.
2. Adjustment of bandpass width by changing the coupling between resonators is proposed by the following methods: that is, distance P between resonators as shown in FIG. 18(a) is changed, a recess 15 on the open face between resonators as shown in FIG. 18(b) is provided, recesses 16, 16 on the side faces between resonators as shown in FIG. 18(c) are provided, or a non-metallized bore 17 between resonators as shown in FIG. 18(d) is provided. However, these methods require long times for changing and adjusting the mold in which the resonator is molded.
3. For resolving the above problems, another method is proposed in which a plurality of resonators 18, 18 . . . 18 are coupled by base plate 19, as shown in FIG. 19. In this method, a certain amount of clearance between resonators 18 and base plate 19 is required to avoid frequency and coupling relation aberrations. But the presence of base plate 19 and the clearance required make the device bulky so it cannot be small like a small bandpass filter of the integral shaped type.